Cells involved in allergic processes employ a variety of signaling cascades to generate inflammatory responses. A subset of these pathways involve receptors coupled to heterotrimeric guanine nucleotide binding proteins (G proteins), including muscarinic, adrenergic, and histaminic receptors. In addition, chemokine receptors, which are also coupled to pertussis-toxin sensitive Gi proteins, have emerged as important mediators of infectious and inflammatory responses, including those to human immunodeficiency virus (HIV). Members of a family of regulators of G-protein signaling (RGS proteins) were recently identified and determined to inhibit G-protein-mediated signaling . At least twenty mammalian RGS proteins share a common sequence motif (RGS domain), which mediates their association with heterotrimeric G-alpha protein alpha subunits of the Gi and Gq classes. This interaction results in a marked enhancement of the GTPase activity of the alpha subunit. Thus, RGS proteins are GTPase-activating proteins (GAPs) for G-alpha subunits, and as such they limit the lifetime of activated G-alpha, resulting in downregulation of responses to G-protein-coupled receptor (GPCR) stimulation, such as MAP kinase activation and the generation of second messengers such as cAMP. Specific residues on RGS4 were found to be necessary for its interaction with Gialpha-1 by the generation of non-functional mutants. These studies were consistent with interpretations of the crystal structure of RGS4 -Gi-alpha-1 that suggested the primary role of RGS proteins was to stabilize the transition state for GTP hydrolysis by G-alpha proteins. Two mutants were also identified which behaved as dominant interfering antagonists of wild type RGS proteins in both in vitro and in vivo assays. RGS4 was localized to select areas of the brain such as the frontal cortex and striatum. High levels of endogenous RGS4 protein were identified in the cytoplasm of neuroblastoma-glioma hybrid NG108 cells. Unlike other several other RGS proteins, which are predominantly membrane-associated, RGS4 is soluble and was shown to translocate to the plasma membrane when co-expressed with a constitutively active, GTPase-deficient Gi-alpha-2 (Q207L) transiently in HEK293 cells.